Recently, optoelectronic device has been succeeded in commercialization and studied intensively for their scale-up and cost-efficient fabrication. Typical examples of such optoelectronic device include organic electroluminescent devices or organic light emitting diodes (OLEDs), which are light emitting devices using a spontaneous light emitting phenomenon caused by coupling between electrons and holes upon the application of electric current to a device including a fluorescence- or phosphorescence-based light emitting layer between an anode and a cathode. Such OLEDs have a simple structure, are obtained by a simple process, and realize high image quality and a broad view angle. Further, they completely realize video images and high color purity, are driven with low power consumption under a low voltage, and thus are suitable for portable electronic appliances.
More particularly, an OLED includes an anode, a hole injection layer, a hole transfer layer, an emitting layer, an electron transfer layer, an electron injection layer and a cathode, stacked successively on a substrate. Herein, the anode is frequently allied of indium tin oxide (ITO) having a low surface resistance and high transmittance. In addition, multiple organic thin films are disposed between the two electrodes as described above to increase the light emitting efficiency and lifespan. Since the organic thin films are very weak to moisture and oxygen in the air, an encapsulating film is formed on the uppermost portion of the device to increase the lifespan thereof.
Many expensive vacuum chambers are required to form such a multilayered OLED with high efficiency and a patterning mask is also required. Moreover, processes for fabricating OLEDs have fundamental limitation in terms of low-temperature operation. For these reasons, it is difficult to scale up OLEDs in their size and to improve cost efficiency. Therefore, there has been a continuous need for developing a novel process to solve the above-mentioned problems.
More recently, many attempts have been made to overcome the above-mentioned problems through the use of a printing process. For example, an inkjet printing process substituting for a known deposition process is differentiated from the deposition process in that it consumes a low amount of materials, shows high efficiency, and allows scale-up and low-temperature operation. Therefore, flexible substrates, such as plastics, may be used in an inkjet printing process, resulting in significant improvement in cost-efficiency. As a result, many Korean and foreign companies and organizations have conducted active research and development of such inkjet printing processes. It is expected that inkjet printing technology is applied to various industrial fields, including electric/electronic, energy, display, bioindustries, etc., and contributes to production of a wide variety of commercial products and improvement in cost-efficiency and eco-friendly characteristics.
Inkjet printing is low-noise, low-cost and non-contact printing technology. Depending on ink spray modes, inkjet printing processes are classified into continuous jet processes and drop-on-demand (DOD) processes. The continuous jet processes performs printing by controlling ink direction through a change in electromagnetic field while ink is sprayed continuously with a pump. The DOD processes spray ink only at a desired moment through electrical signals, and are further classified into piezoelectric inkjet processes generating pressure with a piezoelectric plate that causes dynamic deformation by electricity, and thermal inkjet processes using pressure generated upon the expansion of bubbles produced by heat.
Methods for fabricating OLEDs using such inkjet processes are disclosed in various publications, for example, in T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu and J. C. Stunn,-“Ink-jet Printing of doped Polymers for Organic Light Emitting Devices”, Applied Physics Letters, Vol. 72, No. 5, pp. 519-521, 1998. The known methods frequently use polymeric materials, such as polyvinylcarbazole or polyphenylene vinylene (PPV), but are problematic in that they cause non-uniformity of droplet sizes and degradation of optoelectrical properties as compared to other conventional processes. This may result from the fact that the known processes may not provide an ink composition for inkjet printing that has controllable viscosity, surface tension, solubility, film uniformity after drying, etc., suitable for inkjet processes.
It is required for an ink composition for applying key materials, such as organic materials for light emission, electron transfer or hole transfer, of optoelectronic device, including OLEDs, to inkjet printing processes, to have optimized viscosity, surface tension, solubility, film uniformity after drying, etc. Those properties may affect droplet forming systems, droplet sizes and velocities under a constant pressure. For example, when using a general inkjet system for optoelectronic device, an optimal viscosity of ink is 5-15 cps in view of good ejectability. However, most high-efficiency and high-lifespan compounds commercialized and used currently as key materials of OLEDs have low solubility and small molecular weight, and thus have difficulty in controlling their viscosity to be suitable for inkjet printing processes. For this reason, various additives are used to control the viscosity and tested for ejectability. However, some additives are not removed after drying but still remain in ink to serve as foreign materials, thereby adversely affecting electrical and optical properties, or the like. As a result, it is not possible to maintain a unique color coordination, high efficiency and long lifespan. In addition, it is difficult to control the solubility and molecular weights of organic materials, such as dielectrics, semiconductors and conductors, used as key materials of organic thin film transistor (OTFT) in view of physical properties required for inkjet printing. Conventional inkjet processes using various additives result in degradation of dielectric coefficient, charge transfer and conductivity due to the impurities remaining after the fabrication of devices.
Korean Patent Laid-Open No. 2003-0058767 discloses a method for improving the printability of an organic light emitting layer for OLEDs formed by a roll coating process. The method uses, as a solvent, a mixture containing a first solvent having a solubility of 1 wt/v %, a second solvent having a volatility of 0.1 or less and a third solvent having a surface tension of 30 dynes/cm or less, to prevent solvent evaporation before coating a substrate and to improve solubility and surface tension. However, the above method is merely limited to improvement in solubility characteristics of organic polymer materials used for a specific process, does not allow selection of an adequate combination of solvents depending on different kinds of organic polymer materials, and have difficulty in controlling viscosity suitable for a printing process. Therefore, the method is not applicable to various materials for fabricating optoelectronic device.
As stated above, ink compositions for fabricating optoelectronic device according to the related art have difficulty in controlling viscosity, solubility and film uniformity so that the ink compositions are applicable to printing processes, such as inkjet processes. Therefore, processes for forming films via printing of optoelectronic materials provided as ink have been limited to formation of certain organic light emitting layers. As a result, there has been marked limitation in realizing flexible optoelectronic device, and scale-up and cost-efficient fabrication thereof.